A "potential safety concern" has paused a high-profile international trial of a COVID-19 antibody therapy that was being orchestrated in part by the University of Minnesota and recruiting patients at Hennepin County Medical Center, among numerous other sites.

Eli Lilly, the manufacturer of the experimental synthetic antibody therapy, verified the pause after government officials sent e-mails on Tuesday to researchers at testing sites.

The U's INSIGHT clinical trial network was one of four selected by the National Institutes of Health to recruit patients for the study based on its successful track record of running international studies of medications for HIV and influenza. Network leader James Neaton, a professor at the U's School of Public Health, said there has been a flurry of communication but little clarity yet about the concern that compelled the Data and Safety Monitoring Board to recommend a pause to the federally funded study.

"There's over 300 participants enrolled. We are still following all participants, but not enrolling [new ones] as of 11:15 a.m. this morning," Neaton said in an interview Tuesday afternoon.

The news comes just a day after Johnson & Johnson announced the pause of its coronavirus vaccine trial because of a sick volunteer and a month after AstraZeneca's vaccine trial was halted over concerns about two participants who had fallen ill after getting the company's vaccine. Bloomington-based HealthPartners had been a trial site for the vaccine trial, which has still suspended U.S. recruitment.

The Eli Lilly trial was designed to test the benefits of the antibody therapy on hundreds of people hospitalized with COVID-19, compared with a placebo. All of the study participants also received another experimental drug, remdesivir, which has become commonly used to treat coronavirus patients. It is unclear how many volunteers were sick and what the details of their illnesses were.

The U was activating trial sites in its international network to specifically recruit hospitalized patients, while another clinical network was seeking to provide the antibody therapy, known as LY-CoV555, on an outpatient basis.

Neaton said in August that the goal was to assess whether the antibody therapy helped hospitalized patients achieve "sustained recovery." That means returning home and having sustained good health for two weeks. Researchers also check on the patients 90 days later to assess the safety of the therapy and any long-term complications.

E-mails to HCMC regarding the pause in the trial were not immediately returned Tuesday afternoon.

In large clinical trials, such pauses are not unusual, and illnesses in volunteers are not necessarily the result of the experimental drug or vaccine. Such halts are meant to allow an independent board of scientific experts to review the data and determine whether the event may have been related to the treatment or occurred by chance.

"This is why clinical trials are essential," said Marion Pepper, an immunologist at the University of Washington. "The safety of the product has to be empirically proven."

Without more details on the nature of the illnesses that may have prompted the pause, however, the seriousness of the event remains unclear, Pepper added.

Staff writer Jeremy Olson contributed to this report.